1. Field of the Invention
This invention relates generally to the field of measuring the energy of ionizing electromagnetic radiation and, more particularly, to a vibration resistant high pressure xenon detector for such measurement.
2. Description of the Related Art
There are many fields in which the measurement of the energy of ionizing electromagnetic radiation such as gamma-radiation or X-radiation is desired. For the purposes of this application, the energy of ionizing electromagnetic radiation is equivalent to or a function of the wavelength or frequency of the radiation. Devices to accomplish this measurement are generally referred to as spectrometers. Spectrometry of electromagnetic radiation is a common method of determining the composition of the material from which the radiation emanated. Specifically, gamma-ray spectroscopy will determine the elemental composition of the material from which it emanates. X-Ray spectroscopy will determine the elemental or chemical composition of the material from which it emanates.
Gamma-ray spectrometry is important in all areas of nuclear measurement where gamma-ray energy must be measured or an isotope must be identified. Nuclear reactions or atomic transitions produce gamma-rays or X-rays whose energy identifies the interaction and the constituents in the reaction. Gamma-ray spectrometers typically consist of scintillators (see e.g. Radiation Detection and Measurement, Glenn Knoll, 3rd Ed., pp 219–264) which emit light whose intensity is proportional to the number of electrons ionized in the scintillating material by the gamma ray, semiconductors diode devices (Knoll at 353–404) in which the charge collected is proportional to the number of electrons ionized by a gamma-ray, or a gas-filled device whose charge is proportional to the number of electrons ionized (Knoll at 159–200). Geiger counters are gas-filled detectors whose charge collected is not proportional to the number of electrons ionized, hence are not spectrometers in the current art.
For the purposes of gas-filled spectrometers with which this invention deals, a radiation event occurs within the gas filled chamber when a single gamma ray or single X-ray interacts inside the gas in the chamber within or in proximity to an electro-magnetic field. This event strips off electrons from the gas within the chamber thus creating one or more ions (an ionization event—one radiation event can and usually does create more than one ionization event). The actual physical location of the point where an electron is stripped off is called the “interaction position” which is of importance in determining the spectroscopic information which will be further discussed below. The combination of the influences of all free electrons and ions form a single event which creates the pulse waveform time series influence on the electro-magnetic field.
Gas-filled spectrometers presently suffer from a shortcoming in that the apparent charge collected on the electrodes is dependent on interaction position. (Technically, this is the mirror-charge induced on the electrodes by the motion of the charged particles in the electric field needed to keep the electrodes at their fixed potential—this will be referred to as “collection” as is common in the art.) This “collection” results in a charge induced on the electrodes as the electric charge moves under the influence of the electro-magnetic field. As the charge is induced over time it produces a time series pulse.
As stated above, gamma-rays create electron-ion pairs in the gas. These electron-ion pairs are then counted to determine gamma-ray energy. In a gas, ionized by electromagnetic radiation, ions move much more slowly than the electrons. This leads to a position dependence in apparent charge collected, since the ions are not observed to move or be “collected” on the time scale in which the electrons are observed to move. In other words, the influence the ionization of an atom on an electro-magnetic field is dependant upon and in part a function of the distance of the interaction position from the electrodes that create the electro-magnetic field. This results in two unknown quantities: the energy deposited by the ionizing radiation and the location of the interaction position. Simple measurement of the resulting magnitude of change in the charge of the electrodes is inadequate for spectrometry purposes.
This problem is circumvented in three main ways in the current art. 1) Gas-filled proportional counters are spectrometers by virtue of the fact that they remove the position dependence by electron multiplication near the charge collecting electrode (the anode), so that the observed difference in collection with position is negligible. Any gain element sufficiently near the anode will accomplish this task. 2) Gridded ion chambers solve this problem by limiting the drift distance over which the electrons are observed, by a screening grid, called a Frisch grid. A spectrometer incorporating a Frisch grid is shown in FIG. 1. 3) It has been suggested that a “Luke grid” (See U.S. Pat. No. 5,530,249) could be constructed in which the electron charge is mainly measured by a set of interlocked grids in semiconductor detectors.
The preferred method of addressing this problem in the current state of the art for gas detectors is through the use of a Frisch grid. A current design detector employing a Frisch grid is shown in FIG. 1 wherein an anode 100 is supported within a cylindrical cathode 102. The grid 104 is supported around the anode and a ceramic feed through 106 allows signal transmission from the anode. The Frisch grid is a source of added mechanical complexity and additional electronic noise caused by its capacitance and its microphonic vibration.
It is therefore desirable that spectrometry is performed in the apparatus and the spectroscopic information is extracted from the apparatus in a way that enhances accuracy and reliability.
It is also desirable that a gas spectrometer be provided that is more reliable and sensitive while having greatly reduced sensitivity to mechanical vibration.